Continuous processing of powder coating compositions

ABSTRACT

Systems, apparatus combinations and methods for producing a powder coating are provided wherein a stream of a powder coating precursor including at least one resin and at least one additional powder coating ingredient is contacted with a process fluid effective to reduce the viscosity of the powder coating precursor to allow processing of the powder coating precursor at a lower temperature.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of application,U.S. Ser. No. 08/684,112, filed on Jul. 19, 1996, now U.S. Pat. No.5,766,522. The parent application is hereby incorporated by referenceherein and is made a part hereof, including but not limited to thoseportions which specifically appear hereinafter.

BACKGROUND OF THE INVENTION

This invention relates generally to powder coatings and, moreparticularly, to the continuous processing of powder coatings.

Because of increased environmental concerns, much effort has beendirected to the problem of reducing pollution caused by the evaporationof solvents from paints. These efforts have led to the development ofnew coating technologies which eliminate or at least diminish theemission of organic solvent vapors into the atmosphere. Since themid-1950's, the powder coating technology has been one of the mostsuccessful developments in terms of reducing or eliminating solventvapor emissions.

The use of powder coating compositions can be extremely desirable assuch compositions are essentially free of organic solvents such as areconventionally present in liquid paint systems. Accordingly, economicand social benefits such as reductions in air pollution, energyrequirements, and fire and health hazards can be realized through theuse of powder coatings.

A common technique for applying a powder coating to an object makes useof electrostatic powder spray coating equipment. In such application, acoating powder is dispersed in an airstream and passed through a highvoltage field whereby the coating particles attain an electrostaticcharge. These charged particles are attracted to and deposited on theobject to be coated which is usually at room temperature. Subsequently,the object is placed in an oven and heated whereby the powdermelts/cures to form the desired coating on the object.

U.S. Pat. No. 5,009,367 to Nielsen, U.S. Pat. No. 5,027,742 to Lee etal., and "HIGHER SOLIDS COATINGS ABOVE 80% BY VOLUME," presented at theWater-Borne & Higher Solids Coatings Symposium, Mar. 10-12, 1980, allconcern the spraying of materials using supercritical fluids.

Further, based on U.S. Pat. No. 5,158,986 to Cha et al., U.S. Pat. No.5,334,356 to Baldwin et al., and the article entitled, "NEW ROLES FORSUPERCRITICAL FLUIDS," appearing in Chemical Engineering, March 1994(pages 32-35), it is known to feed fluids, including supercritical CO₂,to an extruder to form an extruded shape of a fluid and polymer plasticmaterial. As disclosed, such extruded material can subsequently beprocessed to form a desired supermicrocellular, foamed material, such asin the form of a sheet.

Conventionally, the manufacture of a powder coating comprisesmelt-mixing a resin, a curing agent, plasticizers, stabilizers, flowaids, pigments, and extenders. Whereas dry blending is commonly used tomake PVC powders under conditions not amenable to the formation of veryfine powders, melt-mixing involves the high speed, high intensity mixingof dry ingredients in a Henschel mixer or the like and then the heatingof the mixture to an elevated temperature (e.g., about 180-250° F.) in acontinuous compounder such as a single or twin screw extruder to achievethorough dispersion of the other ingredients in the resin as the resinmelts, forming a molten mixture. The molten mixture is then cooled toquench the reaction and crushed. Such processing is then generallyfollowed by a sequence of operations which can involve grinding,sifting, separation, and filtering, followed by more sieving.

Such manufacture and processing of coating powders, however, are subjectto a number of shortcomings or difficulties. For example, hightemperature processing of ingredients in a melt extruder can bring aboutpremature reaction of the resin with the curing agent or degradation ofat least some polymer resins.

Additionally, the particles produced as a result of such crushing andgrinding operations are generally substantially non-spherical,irregularly shaped. Such irregularly shaped particles can have anundesirable effect on the uniformity and continuity of any resultingcoating formed on a substrate surface as a result of application andcuring of such a powder coating.

Furthermore, the particles produced by such conventional manufactureprocessing tend to vary greatly in size. Consequently, various particleseparation techniques such as screening and cyclone separation can berequired in order to separate undesirable large and small particles fromthe powder particles having the desired size distribution. The powderparticles which are undesirably sized must then typically be downgradedor otherwise disposed of.

In the past, various approaches have been proposed in order to overcomeor minimize some of the above-identified problems.

U.S. Pat. No. 5,207,954 to Lewis et al. discloses a method of making athermosettable, coreactable particulate powdered composition of a firstcopolymer of an olefinically unsaturated monomer having at least onefunctional group and at least a second copolymer of an olefinicallyunsaturated monomer having at least one functional group which isreactive with the functional group of the first copolymer. Aqueousdispersions containing the coreactive polymers are disclosed as beingspray dried to produce copolymeric particles which are substantiallyuniform and spherical in shape.

U.S. Pat. Nos. 4,582,731 and 4,734,451, both to Smith, disclose methodsand apparatus for the deposition of thin films and the formation ofpowder coatings through the molecular spray of solutes dissolved inorganic and supercritical fluid solvents. The examples disclose theapplication of single component films to substrate surfaces. Thesepatents do not appear to disclose coating materials composed of multiplecomponents or materials, or the processing thereof.

U.S. Pat. No. 5,290,827 to Shine concerns a process for preparing ahomogeneous blend of otherwise thermodynamically immiscible polymers,rather than resins with or without a curing agent. In accordance withthe disclosure, mixtures of polymers are dissolved under pressure insupercritical fluid solvents and then expanded through a fine nozzle. Asthe supercritical fluid solvent evaporates, the polymer mixture isdisclosed as depositing as a substantially homogeneous blend.

U.S. Pat. No. 5,399,597 to Mandel et al. discloses a batch process forpreparing powder coating materials whereby at least some of theabove-identified problems are sought to be minimized or avoided. Inaccordance with the process thereof, different first and second organicmaterials and a supercritical fluid are mechanically agitated in a firstcontainer. The contents of the first container are then discharged intoa second container, maintained at a lower pressure than the firstcontainer, and in which substantially all of the first and secondorganic materials are collected.

Such batch processing can suffer from a number of shortcomings. Forexample, batch processing can undesirably result in long cycle timeswhich, for example, can cause undesired polymerization of fast curingpowder coating compositions. Further, batch processing can lead toproduct inconsistencies, such as inconsistencies in product propertiessuch as viscosity and particle size, due to variations in processingconditions such as pressure and mixing time over the course of a batchrun. Still further, large batch runs will typically necessitate the uselarge processing vessels. Large processing vessels can in turn proveundesirably time consuming to properly clean between runs for or withdifferent product compositions. In addition, in such batch processing itcan be difficult to maintain high pressure seals such as typicallyrequired to contain supercritical process fluids.

Further, U.S. Pat. No. 5,399,597 emphasizes that with the processdisclosed therein, solubilization of components in the supercriticalfluid is undesirable as such solubilization would unavoidably result inlose of material upon transfer from the process vessel to the productreceiving vessel. The patent teaches the avoidance of such undesirableresults through the selection of materials which are not soluble in thesupercritical fluid at the operating conditions.

SUMMARY OF THE INVENTION

A general object of the invention is to provide improved processing ofpowder coatings.

A more specific objective of the invention is to overcome one or more ofthe problems described above.

The general object of the invention can be attained, at least in part,through the production of a powder coating by a method wherein a streamof a powder coating precursor is contacted with a process media fluideffective to reduce the viscosity of the powder coating precursor streamto allow processing of the powder coating precursor stream at a lowertemperature. The powder coating precursor stream includes powder coatingingredients including at least one resin and at least one additionalpowder coating ingredient. The process media fluid includes a processmedia material in the form of a fluid selected from the group consistingof supercritical fluids and liquified gases.

In one particular embodiment, the process media fluid is effective toplasticize at least one of the resin and additional powder coatingingredient.

In another particular embodiment, the process media fluid is asupercritical fluid effective to wholly or partially dissolve at leastone of the resin and additional powder coating ingredient.

The prior art fails to provide systems, apparatus combinations andmethods for continuous process production of powder coatings,particularly the production of powder coatings having greater uniformityin one or more properties or characteristic such as particle size,shape, color, gloss and cure rate.

The invention further comprehends a method for producing a powdercoating wherein powder coating raw materials are fed to and processed ina continuous extruder. The powder coating raw materials fed to andprocessed in the continuous extruder include at least one resin and atleast one additional powder coating ingredient, with the extruderprocessing being effective to disperse the at least one additionalingredient with the at least one resin to form an extrudate product. Themethod of the invention includes the step of adding a process mediafluid comprising a process media material in the form of a fluidselected from the group consisting of supercritical fluids and liquifiedgases to a process stream of at least one of the following:

a.) the raw materials fed to the continuous extruder;

b.) the raw materials processed in the continuous extruder; and

c.) the extrudate product of the continuous extruder.

The addition of the process media fluid being effective to reduce theviscosity of the selected process stream to allow processing of theprocess stream at a lower temperature.

The invention still further comprehends a method for producing a powdercoating wherein a premixed blend of powder coating raw materials areextruded to form an extrudate product. In one embodiment, the premixedblend of powder coating raw materials includes at least onethermosettable resin and at least one curing agent for the at least onethermosettable resin. A stream of the extrudate product is then fedthrough a melt pump to form a stream of extrudate product at increasedpressure. The stream of extrudate product at increased pressure is thenspray dried to form the powder coating.

In accordance with this method, at least one of the blend of powdercoating raw materials undergoing extrusion and the stream of extrudateproduct at increased pressure is contacted with a process media fluidselected from the group consisting of supercritical fluids and liquifiedgases. The process media is effective to reduce the viscosity of thematerials of the selected process stream to allow processing at a lowertemperature.

The invention also comprehends systems for producing a powder coating.In accordance with one embodiment, the powder coating producing systemof the invention includes a continuous extruder wherein powder coatingraw materials including at least one resin and at least one additionalpowder coating ingredient are fed and processed to disperse the at leastone additional ingredient with the at least one resin to form anextruded coating precursor stream.

The system also includes a source of a process media material. Theprocess media material achieves a fluid condition within the processingsystem and is effective to reduce the viscosity of the powder coatingprecursor stream to allow processing of the powder coating precursorstream at a lower temperature.

The system further includes means for adding such process media materialto at least one of the following:

a.) the raw materials fed to the continuous extruder;

b.) the raw materials processed in the continuous extruder; and

c.) the extruded product of the continuous extruder, and

means for forming and separating the powder coating from the processmedia material.

In particular embodiments, such means for forming and separating thepowder coating from the process media material can take various formsincluding: spray drying (including spray drying into a reclamationbooth), formation of a foam or friable mass suitable for subsequentgrinding or the like reduction into a desired powder form, and sprayinginto a solution.

The invention also comprehends a system for producing a powder coatingincluding a continuous extruder, a melt pump, a source of a processmedia material. In the continuous extruder, powder coating raw materialsincluding at least one thermosettable resin and at least one curingagent for the at least one thermosettable resin are fed and processed todisperse the at least one curing agent with the at least onethermosettable resin to form a molten extruded powder coating precursor.The molten extruded powder coating precursor is processed through themelt pump to form a powder coating precursor stream of increasedpressure. The source of a process media material contains a processmedia material which, within the processing system, is in a fluidcondition and effective to reduce the viscosity of the powder coatingprecursor stream to allow processing of the powder coating precursorstream at a lower temperature.

The system further includes means for adding such process media materialto the powder coating raw materials in the continuous extruder and aspray drier to form and separate the powder coating from the processmedia material.

As used herein, references to a "supercritical fluid" are to beunderstood to refer to a material that is at a temperature and pressuresuch that it is at, above, or slightly below its critical point.

As used herein, the "critical point" is the transition point at whichthe liquid and gaseous states merge into each other and represents thecombination of the critical temperature and critical pressure for agiven substance.

The "critical temperature," as used herein, is defined as thetemperature above which a gas cannot be liquefied by an increase inpressure.

The "critical pressure," as used herein, is defined as that pressurewhich is just sufficient to cause the appearance of two phases at thecritical temperature.

As used herein, references to a "liquified gas" are to be understood torefer to a material which is a liquid but which at standard conditionsof temperature and pressure is gas.

The term "generally spherical particles," as used in the context of thisinvention, encompasses particles having true spherical shapes to thosehaving near spherical shapes. Near spherical shapes include ovoid shapedparticles; particles having open or closed bulbous protuberances, suchprotuberances may or may not be generally spherically shaped; andparticles having cellular portions therein. Such cellular portions mayextend or be contained internally and/or externally of the major surfaceof the particle and may be open or closed.

The term "cellular," as used in the context of this invention, meanshaving at least some hollow portion or portions.

Other objects and advantages will be apparent to those skilled in theart from the following detailed description taken in conjunction withthe appended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic flow diagram of a powder coatingprocessing system in accordance with one embodiment of the invention.

FIG. 2 is a simplified schematic flow diagram of a powder coatingprocessing system in accordance with an alternative embodiment of theinvention.

FIG. 3 is a simplified schematic flow diagram of a powder coatingprocessing subsystem in accordance with one embodiment of the invention.

FIG. 4 is a simplified schematic flow diagram of a powder coatingprocessing subsystem in accordance with another embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is schematically shown a system, generallydesignated by the reference numeral 10, for the processing of a powdercoating in accordance with the invention. The system 10 includes asource 12 of powder coating raw materials, such as described below, fromwhich a flow stream 14 of raw materials is passed to a raw material feedsystem 16 to form a flow stream 20 containing a premixed blend ofresin-containing powder coating raw materials.

The flow stream 20 is fed to a continuous extruder 22, such as a twinscrew extruder, wherein the premixed blend of powder coating rawmaterials are extruded. In practice, the residence time of the materialswithin such an extruder is generally less than about 2 minutes andtypically in a range of about 30-45 seconds.

The system 10 additionally includes a source 24 of a process media. Asdescribed in greater detail below, the process media comprises one ormore materials which achieve a fluid condition, i.e., a supercriticalfluid or a liquified gas, within the processing system. Such processmedia fluid is effective in reducing the viscosity of powder coatingprecursor materials or at least selected components of a powder coatingprecursor composition, especially the powder coating resin. In practice,such viscosity reduction can be realized via plasticization,solubilization or partial solubilization of at least particularlyselected components of a powder coating precursor composition.

It will be appreciated that such process media materials can bevariously added such as by the original addition of the material in suchfluid condition, i.e., the material is originally added as asupercritical fluid or a liquified gas, or by the addition of materialin a form which sequentially attains the desired fluid condition. Forexample, originally a liquified gas can be added with such materialsubsequently attaining a supercritical fluid condition in the processingsystem, such as may be desired.

A flow stream 26 of such a process media is passed through a pressureboosting device 30, such as a pump or a compressor, to form a flowstream 32.

With the opening of the valve 34a, at least a portion of the processmedia flow stream 32, designated as the flow stream 32a, is passed andadded to the materials being processed in the continuous extruder 22.For example, the flow stream 32a of the process media can be directlyinjected into the barrel portion of such a continuous twin screwextruder. As the materials being processed in the continuous extruderwill be undergoing high sheer mixing, the intake of the process mediafluid by such materials can be most significant at this position in theprocessing.

As a result of such process media fluid intake and in comparison tosimilar processing without such process media intake, lower viscositiesare observed, with subsequent lowering of the processing temperature. Itwill be appreciated that with sufficient lowering of viscosity, theprocess product can be atomized or, alternatively, isolated as a friableor foam mass which is easily reduced to a powder product. For example,the product out of the extruder may be suitable for immediate furtherprocessing such as grinding to form the desired powder product form.

It will also be appreciated that in such processing, the material massbeing processed can serve to form the seals required in and for suchprocessing apparatus, thus avoiding the problems associated with priorart high pressure operation, such as identified above.

If necessary or desired and as shown in the embodiment of the inventionillustrated in FIG. 1, a flow stream 60 of extruded product, such as inthe form of a molten material, is passed to a melt pump 62 through whichthe extruded powder coating precursor is processed to form a powdercoating precursor flow stream 64 of increased pressure to facilitate andpermit desired subsequent processing, such as described below. Such amelt pump can, for example, take the form a diaphragm pump, an extruderor, in one preferred embodiment of the invention, a gear pump. Inpractice, gear pumps are well suited to handle the high torquesresulting from use with high viscosity materials such as resins,fillers, etc. such as are commonly present in powder coatingcompositions.

It will further be appreciated that detrimental viscosity reductionwithin the extruder can result in either or both poor mixing and flowreversal within the extruder. To avoid such detrimental viscosityreduction, it may be necessary or desirable to limit the amount ofprocess media fluid added to the extruder. However, the addition offurther of the process media fluid can be desired where, for example,the viscosity of the materials being processed has not been sufficientlyreduced to easily permit or allow the subsequent atomization of thepowder coating precursor material through a spray nozzle or air-assistednozzle, such as to assist in atomization, or the static mixing orparticle size reduction of suspended solids, such as via aMICROFLUIDIZER process apparatus by Microfluidics InternationalCorporation of Newton, Mass., for example, as later described herein.

Thus, while in the system described above the process media is disclosedas being added to the materials being processed in the continuousextruder 22, it will be appreciated that such process media can, ifdesired, be additionally or alternatively added at other locations inthe system. For example, alternatively or in addition to the passage andaddition of at least a portion of the process media to the materialsbeing processed in the continuous extruder 22, a portion of the processmedia flow stream 32, designated as the flow stream 32b, can with theopening of a valve 34b, if desired, be passed and added to the powdercoating precursor flow stream 64 of increased pressure to form a powdercoating precursor flow stream, designated 66, containing additionalprocess media fluid. In general, the powder coating precursor flowstream 66 is of the form of a fluid solution.

The powder coating precursor flow stream 66 is then passed to one ormore continuous fluid mixers 70, such as static in-line mixers,resulting in further mixing of the various components of the powdercoating precursor and, desirably, facilitate reduction of the viscosityof the materials streams being processed to form a flow stream 72.

The flow stream 72 is directed to a receiver vessel 74, such as througha spray nozzle or air-assisted nozzle 76, and released to lower,desirably atmospheric pressure. In one embodiment of the invention, thecontinuous high pressure in the system resulting from the inclusion ofthe above-described melt pump 64 ensures the flow stream 72 can properlybe passed through the nozzle 76.

As a result of such release to such lower, desirably atmosphericpressure, either or both supercritical fluid and liquified gas processmedia contained in the flow stream will desirably rapidly, preferablyimmediately, gasify. The gasification of the process media fluiddesirably results in the foaming or atomization of the remaining powdercoating precursor material.

If desired, an external atomizing force, such as high pressure air, canat this point be injected at the nozzle to facilitate furtheratomization and formation of particles of the remaining powder coatingprecursor material.

From the receiver vessel 74, a flow stream 80 of powder coating materialis passed to a final processing step 82 wherein, if necessary ordesired, the powder coating material can be further ground or screenedto produce powder coating particles of the appropriately desireddimensions. In practice, such further grinding and screening willtypically only be required or needed when more than about 1 to 2 percentof powder coating material discharged from the receiver vessel 74exceeds the desired particle size range.

Additionally, postblending of additives such as fumed silicas andaluminum oxide, such as to improve powder flow, eliminate caking of thepowder composition or both, or additives to produce desired specialeffects such as hammertones or metallic coating, for example, can bedone, if desired.

A flow stream 84 of finished powder coating particles is passed to afinal product collecting vessel 86 for subsequent packaging andhandling.

Referring again to the receiver vessel 74, there is formed therefrom aflow stream 88 of volatiles including recovered process media. Ifdesired, this flow stream 88 can be passed to a recovery unit 90, suchas a condenser and/or a separator, whereby the process media material isgathered and passed as a flow stream 92 to a pressure boosting device94, such as a compressor or liquid pump, to form a flow stream 96 forreturn to the process media source 24.

Thus, the invention provides for simple and effective removal of theprocess media fluid from the powder coating precursor. As a result,while the presence and use of the process media fluid in accordance withthe invention can advantageously facilitate the processing andpreparation of powder coatings, such process media fluids willadvantageously not detrimentally affect the characteristics andproperties of powder coatings so processed.

While in the system as described above the process media fluid has beendescribed as being added to either or both the materials being processedin the continuous extruder 22 and the powder coating precursor resultingfrom a continuous extruder, it will be appreciated that such processmedia fluid can, if desired, be additionally or alternatively added atstill other points in the system.

For example, with the opening of a valve 34c, a portion of the processmedia flow stream 32, designated as the flow stream 32c, can, ifdesired, be passed and added to the raw materials in the continuous rawmaterial feed system 16. Such addition of the process media fluid withthe raw materials being processed can be especially advantageous inconjunction with those temperature sensitive raw materials which permitprocessing, such as occurs within an extruder, at only relative lowprocessing temperatures.

Thus, in practice, extrusion processing at or below the softening pointtemperature of the resin can be significant, with processing temperaturereductions of at least about 10-20° F. or more, preferably at leastabout 20-40° F. or more, below the temperature at which the comparablecomposition without the process media fluid could be processed can beparticularly significant and desirable, as described below.

The introduction of the process media fluid with the raw materials canserve to reduce the viscosity of the stream of materials being processedprior to the materials being extruded. As a result of such reducedviscosity, the amount of work input to the materials being processedduring such extrusion processing can be substantially reduced, thusreducing the temperatures realized within such an extruder. As a result,raw materials having relatively low cure temperatures can now bepractically utilized in powder coating formulations.

Alternatively or in addition, with the opening of a valve 34d, a portionof the process media fluid flow stream 32, designated as the flow stream32d, can, if desired, be directed at and externally applied to suchseals and valves as may be present in the melt pump 62 to assist inkeeping such surfaces free of the powder coating materials beingprocessed. It is to be appreciated that such application of the processmedia fluid can serve to prevent undesired curing of thermosettableresins in areas such as the pump seal area.

Process media useful in the practice of the invention are generallyeffective in reducing the viscosity of powder coating precursormaterials or at least selected components of a powder coating precursorcomposition, especially the powder coating resin. In practice, suchviscosity reduction can be realized via plasticization, solubilizationor partial solubilization of at least particularly selected componentsof a powder coating precursor composition.

In particular, in one embodiment of the invention such viscosityreduction as a result of addition of the process media fluid is realizedwithout any significant solubilization of the resin or other compositioncomponent.

For example, for epoxy and polyester powder coating resins a processmedia fluid of carbon dioxide can serve to plasticize theresin-containing composition. Also, in such processing, the processmedia fluid can be added in sufficiently small amounts to achievedesired processing benefits such as resin softening and temperaturereduction without significantly dissolving such resin materials. As aresult, the amount of process media fluid required can be substantiallyminimized thereby further improving the economics of such processing.

In another embodiment, such viscosity reduction as a result of additionof the process media fluid has associated therewith significantsolubilization of the resin or other composition component such as toform low viscosity solutions which, for example, can be sprayable suchas to form regular spheres.

As described above, the process media of the invention can comprise oneor more materials which achieve a fluid condition, i.e., a supercriticalfluid or a liquified gas, within the processing system. In someparticular embodiments of the invention, at least one or more of suchprocess media materials attains a supercritical fluid state within theprocessing system. In some particular embodiments of the invention, atleast one or more of such process media materials attains a liquifiedgas fluid state within the processing system. While in some embodimentsat least one or more of such process media materials attains asupercritical fluid state within the processing system and at least oneor more of such process media materials attains a liquified gas fluidstate within the processing system.

Examples of compounds which can be used as such process media fluids aregiven in Table I. Others will occur to those skilled in the art.

                  TABLE 1                                                         ______________________________________                                        EXAMPLES OF PROCESS MEDIA FLUIDS                                                                       Critical                                                Boiling Temper- Critical Critical                                             Point ature Pressure Density                                                 Compound (° C.) (° C.) (atm) (g/cm.sup.3)                     ______________________________________                                        CO.sub.2     -78.5   31.3      72.9  0.448                                      NH.sub.3 -33.35 132.4 112.5 0.235                                             H.sub.2 O 100.00 374.15 218.3 0.315                                           N.sub.2 O -88.56 36.5 71.7 0.45                                               Methane -164.00 -82.1 45.8 0.2                                                Ethane -88.63 32.28 48.1 0.203                                                Ethylene -103.7 9.21 49.7 0.218                                               Propane -42.1 96.67 41.9 0.217                                                Pentane 36.1 196.6 33.3 0.232                                                 Benzene 80.1 288.9 48.3 0.302                                                 Methanol 64.7 240.5 78.9 0.272                                                Ethanol 78.5 243.0 63.0 0.276                                                 Isopropanol 82.5 235.3 47.0 0.273                                             Isobutanol 108.0 275.0 42.4 0.272                                             Chlorotriflu- 31.2 28.0 38.7 0.579                                            oromethane                                                                    Monofluoro- 78.4 44.6 58.0 0.3                                                methane                                                                       Toluene 110.6 320.0 40.6 0.292                                                Pyridine 115.5 347.0 55.6 0.312                                               Cyclohexane 80.74 280.0 40.2 0.273                                            m-Cresol 202.2 433.0 45.0 0.346                                               Decalin 195.65 391.0 25.8 0.254                                               Cyclohexanol 155.65 356.0 38.0 0.273                                          o-Xylene 144.4 357.0 35.0 0.284                                               Tetralin 207.57 446.0 34.7 0.309                                              Aniline 184.13 426.0 52.4 0.34                                                1,1,1,2 Tetra- -26.1 101.1 40.1 0.515                                         fluoroethane                                                                ______________________________________                                    

In addition, near supercritical liquids demonstrate solubilitycharacteristics and other properties similar to those of supercriticalfluids. The solute may be a liquid at the supercritical temperatures,even though it is a solid at lower temperatures. In addition, it hasbeen demonstrated that fluid "modifiers" can often alter supercriticalfluid properties significantly, even in relatively low concentrations,greatly increasing solubility for some compounds. These variations areconsidered to be within the concept of a supercritical fluid as used inthe context of this invention.

Viscosity reduction achieved through the addition of the process mediafluid in accordance with the invention can have significant beneficialprocessing advantages. For example, in one embodiment, the addition ofthe process media fluid is effective to form a powder coating precursormaterial which can be sprayed through a nozzle to provide a powdercoating material which requires no additional grinding or screeningprocessing.

In another embodiment, the addition of the process media fluid iseffective to reduce the processing temperature of the materials beingprocessed. For example, the addition of the process media fluid can beeffective to reduce the processing temperature in the system continuoustwin screw extruder. Thus, through the addition of a process media fluidin accordance with the invention, the extrusion temperature of epoxyresin-based powder coating precursor can be reduced from about 180° F.or more to about 100° F., for example. As a result of the utilization ofsuch lower processing temperatures, the range of compositions isincreased as materials unsuitable for use at such prior higherprocessing temperatures can now be utilized.

In practice, such addition of the process media fluid permits theprocessing temperature to be reduced below, preferably about 10-20° F.below, and in some cases more preferably at least about 20-40° F. below,the softening temperature of the particular resin of the powder coatingcomposition under preparation.

In addition to being effective in reducing the viscosity of powdercoating precursor materials or at least selected components of a powdercoating precursor composition, especially the powder coating resin, theprocess media fluid contacted with the coating precursor will preferablybe of a composition or form which facilitates subsequent removal of theprocess media material prior to formation of the final powder coatingparticles. For example, supercritical fluid process media will typicallyflash to a gas when exposed to reduced or atmospheric pressure.

It is to be appreciated and understood that the means for forming andseparating the powder coating from the process media material can takevarious forms dependent on the specific processing needs. Thus, meansfor forming and separating the powder coating from the process mediamaterial in accordance with the invention can include: spray drying(including spray drying into a reclamation booth), formation of a foamor friable mass suitable for subsequent grinding or the like reductioninto a desired powder form, and spraying into a solution.

In connection therewith, although the addition of a process mediamaterial in accordance with the invention can serve to reduce theviscosity of the material being processed, the viscosity of at leastcertain materials so processed may remain sufficiently high that theprocessing of such materials through a spray nozzle may remaindifficult.

Turning to FIG. 3, there is schematically illustrated a processingsubsystem 300 for the formation and separation of a powder coating froma process stream in accordance with one embodiment of the invention.More specifically and as shown, a process stream 302, such as theabove-described flow stream 72, is passed through a container wall 304and out via a heated nozzle 306 to form a spray 310 of powder coatingparticles. It will be appreciated that the nozzle 306, which includesthe process material flow conduit 312 adjacent the nozzle outlet 314,can be of various forms including, for example, a nozzle heated by meansof a hot oil or electric coil 316, for example. Such heating of thenozzle can serve to increase the solubility of the process media in thematerial being processed, thus further reducing the viscosity ofmaterial being processed sufficiently to allow atomization of theprocess material and the obtaining of spherical particles upon sprayingof the material being processed. In such a processing subsystem, heat isgenerally applied at the point of atomization or formation of the powdercoating particles.

While alternative methods of increased or further viscosity reduction,such as through the addition of a co-solvent, are discussed in greaterdetail below, it will be appreciated that such addition of a co-solventcan be undesired because of the increased costs associated therewith,e.g., the cost of the co-solvent itself and the need or desire torecycle and process such a co-solvent. Thus, it will be appreciated thatsuch a processing system wherein heat is generally applied at the pointof atomization or formation of the powder coating particles can offer orprovide certain processing advantages.

Reference is now made to FIG. 4 which schematically illustrates aprocessing subsystem 400 for the formation and separation of a powdercoating from a process stream in accordance with an alternativeembodiment of the invention. More specifically and as shown, a processstream 402, such as, for example, the above-described extruded productflow stream 60, powder coating precursor flow stream 64 or flow stream72, is passed to a mill, grinder or the like 404. If necessary ordesired, a flow stream 406 of a cryogen such as liquid nitrogen is addedto the mill 404 and a flow stream 410 of powder coating material ispassed from the mill 404.

It will be appreciated that such addition of a cryogen may be desired orrequired in order to permit or facilitate the ready grinding ofmaterials such as thermoplastic resin-based powder coating compositions,as commonly associated with commercial grinding of such materials.

The grinding of powder coating process streams, in accordance with theinvention, is described in greater detail in connection with Examples,below.

In addition, the process media used in the practice of the inventionpreferably are relatively inexpensive, recyclable, nontoxic andnon-reactive with powder coating composition ingredients.

Carbon dioxide as either a liquified gas or a supercritical fluid is apreferred process media fluid for use in the practice of the invention.The solvency of supercritical carbon dioxide is similar to that of alower hydrocarbon (e.g., butane, pentane, or hexane) and, as a result,one can consider supercritical carbon dioxide as a replacement for thehydrocarbon diluent portion of a conventional solvent borne coatingformulation.

In practice, such a process media are added ranging from about 0.1 toabout 99 or more parts of process media to base resin. For example,supercritical and liquid CO₂ are typically added in relative amounts ofabout 10 to about 90% by weight of the resin.

It is further to be appreciated that it may sometimes be desirable toemploy one or more co-solvents in addition to the process media. Forexample, the inclusion of a co-solvent may be desired where the additionof the process media, such a liquified or supercritical CO₂, does not byitself reduce the viscosity of the powder coating precursor materials orat least selected components of a powder coating precursor composition,especially the powder coating resin to the extent desired or requiredfor the desired further processing. The addition of a co-solvent mayalso be desired in order to more fully or completely dissolve selectedcomponents of the coating powder mixture.

Co-solvent(s) suitable for the practice of this invention generallyinclude any solvent or mixture of solvents which is miscible with theprocess media fluid and is a good solvent for a powder component.Additionally, desired co-solvents are generally significantly unreactivewith the powder coating composition materials and are relatively easilyremoved, such as by drying or extraction processing, from the powdercoating precursor.

Solubility parameters may be taken into account in the choice of thesolvent. It is recognized that some organic solvents, such ascyclohexanol, have utility as both conventional solvents and as aprocess media. As used herein, the term "co-solvent" does not includesolvents in the supercritical or liquified gas state.

Among suitable co-solvents are organic solvents such as:tetrahydrofuran, ketones such as acetone, methyl ethyl ketone, methylisobutyl ketone, mesityl oxide, methyl amyl ketone, cyclohexanone andother aliphatic ketones; esters such as methyl acetate, ethyl acetate,alkyl carboxylic esters, methyl t-butyl ethers, dibutyl ether, methylphenyl ether and other aliphatic or alkyl aromatic ethers; glycol etherssuch ethoxyethanol, butoxyethanol, ethoxypropanol, propoxyethanol,butoxypropanol and other glycol ethers; glycol ether ester such asbutoxyethoxy acetate, ethyl ethoxy propionate and other glycol etheresters; alcohols such methanol, ethanol, propanol, 2-propanol, butanol,amyl alcohol and other aliphatic alcohols; aromatic hydrocarbons such astoluene, xylene, and other aromatics or mixtures of aromatic solvents;and nitro alkanes such as 2-nitropropane. Generally, co-solventssuitable for this invention must have the desired solvencycharacteristics as aforementioned and also the proper balance ofevaporation rates so as to insure good powder formation. A review of thestructural relationships important to choice of solvent or solvent blendis given by Dileep et al., Ind. Eng. Chem. (Product Research andDevelopment) 24, 162, 1985 and Francis, A. W., J. Phys. Chem. 58, 1009,1954.

In practice, such co-solvents are typically added in relative amounts ofabout zero to about 50% by weight of the total mass of the composition.

As described above, viscosity reduction as a result of addition of theprocess media fluid has associated therewith in one embodiment of theinvention significant solubilization of the resin or other compositioncomponent such as to form low viscosity solutions. For example, aprocess media fluid of supercritical carbon dioxide and a co-solventsuch as tetrahydrofuran or methyl ethyl ketone, for example, can beeffective to dissolve the powder coating resin or other coatingcomposition resin.

FIG. 2 illustrates schematically a system, generally designated by thereference numeral 110, for the processing of a powder coating inaccordance with another embodiment of the invention. As will bedescribed in greater detail below, the system 110 provides for theaddition of both a co-solvent and a supercritical fluid process media.

The system 110, similar to the system 10 described above, includes asource 112 of powder coating raw materials from which a flow stream 114of raw materials is passed to a raw material feed system 116 to form aflow stream 120 containing a premixed blend of powder coating rawmaterials. The flow stream 120 is fed to a continuous extruder 122wherein the premixed blend of powder coating raw materials, such asdescribed herein, are extruded.

As with the system 10, described above, the system 110 additionallyincludes a source, herein designated 124, of a process media. In oneparticular embodiment of the invention, a flow stream 126 of thisprocess media is passed through a pressure boosting device 130, such asa pump or a compressor, such that the process media achieves asupercritical state, forming a flow stream 132. In particular, suchsupercritical fluid is effective to plasticize at least selectedcomponents of a powder coating precursor composition.

With the opening of a valve 134a, at least a portion of thesupercritical fluid process media flow stream 132, designated as theflow stream 132a, is passed and added to the materials being processedin the continuous extruder 122.

The system 110 additionally include a source 135 of at least oneco-solvent, such as described herein and desirably in the form of aliquid.

Such addition of co-solvent can be desired where, for example, the addedprocess media fluid is unable to alone provide the desired amount ofplasticization to at least selected components of the powder coatingprecursor composition.

In one particular embodiment of the invention, the process media is asupercritical fluid, such as CO₂, and a co-solvent, such astetrahydrofuran or methyl ethyl ketone, for example, appropriatelyselected from the above-provided listing of solvent materials is added.

As identified and described in commonly assigned patent application Ser.No. 08/662,104, filed Jun. 14, 1996 as a file wrapper continuationapplication of patent application Ser. No. 08/354,308, filed Dec. 12,1994, the disclosures of which are fully incorporated herein byreference, such co-solvent addition can result in the formation ofcellular, generally spherical coating powder particles such as can bedesirable in the formation of thin film coatings.

In one particular embodiment of the invention, a flow stream 136 of theco-solvent is passed through a pressure boosting device 140, such as apump or a compressor, to form a flow stream 142.

The flow stream 142 or portions thereof can then be appropriatelydirected so that the co-solvent is added to the desired process streams.For example, with the opening of a valve 144a, at least a portion of theco-solvent flow stream 142, designated as the flow stream 142a, ispassed and added to the materials being processed in the continuousextruder 122.

As with the above-described system 10, the extruder forms a stream ofextruded product, herein designated as the flow stream 160. The flowstream 160 of the extruded product, such as in the form of a moltenmaterial, is passed to a melt pump 162 through which the extruded powdercoating precursor is processed to form a powder coating precursor flowstream 164 of increased pressure. As described above, such a melt pumpcan, for example, take the form a diaphragm pump, an extruder or, in onepreferred embodiment of the invention, a gear pump.

With the opening of a valve 134b, a portion of the supercritical fluidprocess media flow stream 132, designated as the flow stream 132b, ispassed and added to the powder coating precursor flow stream 164 ofincreased pressure.

Additionally, if desired, with the opening of a valve 144b, a portion ofthe co-solvent flow stream 132, designated as the flow stream 132b, ispassed and added to the powder coating precursor flow stream 164 ofincreased pressure.

Such addition of one or more of the supercritical fluid process mediaand liquid co-solvent forms a powder coating precursor flow stream,designated 166, containing some or additional liquid co-solvent and/orsupercritical fluid process media. As described above, such addition offurther of the liquid co-solvent and the supercritical fluid processmedia can be desired where, for example, the viscosity of the materialsbeing processed has not been sufficiently reduced to easily permit orallow the subsequent atomization of the powder coating precursormaterial through a nozzle. In general, the powder coating precursor flowstream 166 is of the form of a fluid solution.

The powder coating precursor flow stream 166 is then passed to one ormore continuous fluid mixers 170, such as static in-line mixers,resulting in further mixing of the various components of the powdercoating precursor and, desirably, facilitate reduction of the viscosityof the materials streams being processed to form a flow stream 172.

The flow stream 172 is directed to a receiver vessel 174, such asthrough a spray or air-assisted nozzle 176, and released to atmosphericpressure. As a result, the process media contained in the flow streamwill immediately gasify and cause foaming and atomization of theremaining powder coating precursor material.

If desired, and external atomizing force, such as high pressure air, canat this point be injected into the receiving vessel to facilitatefurther atomization and formation of particles of the remaining powdercoating precursor material.

From the receiver vessel 174, a flow stream 180 of powder coatingmaterial is passed to a final processing step 182 wherein, as describedabove, if necessary or desired, the powder coating material can befurther ground or screened to produce powder coating particles of theappropriately desired dimensions.

Further, as also described above., postblending of additives can also bedone, if desired.

A flow stream 184 of the finished powder coating particles is passed toa final product collecting vessel 186 for subsequent packaging andhandling.

Referring again to the receiver vessel 174, there is formed therefrom aflow stream 188 of volatiles including recovered fluid process media andco-solvent. This flow stream 188 is passed to a recovery unit 190, suchas a condenser and/or a separator, whereby the process media material isseparated from the co-solvent. The process media is passed as a flowstream 192 to a pressure boosting device 194, such as a compressor orliquid pump, to form a flow stream 196 for return to the process mediasource 124. The co-solvent in turn is passed as a flow stream 198 to theprocess media source 135.

While in the system as described above, the process media fluid andco-solvent have been described as being added to either or both thematerials being processed in the continuous extruder 122 and the powdercoating precursor resulting from a continuous extruder, it will beappreciated that such process media fluid and co-solvent can, ifdesired, be additionally or alternatively added at still other points inthe system.

For example, with the opening of a valve 134c, a portion of the processmedia flow stream 132, designated as the flow stream 132c, can, ifdesired, be passed and added to the raw materials in the continuous rawmaterial feed system 116.

Similarly, with the opening of a valve 144c, a portion of the co-solventflow stream 142, designated as the flow stream 142c, can, if desired, bepassed and added to the raw materials in the continuous raw materialfeed system 116.

Alternatively or in addition, with the opening of a valve 134d, aportion of the process media fluid flow stream 132, designated as theflow stream 132d, can, as described above, be directed at and externallyapplied to such seals and valves as may be present in the melt pump 162.

Similarly, with the opening of a valve 144d, a portion of the co-solventflow stream 142, designated as the flow stream 142d, can, if desired, bedirected at and externally applied to such seals and valves as may bepresent in the melt pump 162 such as to assist in keeping such surfacesfree of the powder coating materials being processed.

Examples of powder coating raw materials suitable for use in the presentinvention include thermoplastic and thermoset base resins.

Thermoplastic resins suitable for use in the coating powders of thisinvention must melt and flow out to a thin film within a few minutes atapplication temperatures of from 200° C. to 300° C. without significantdegradation. Examples of suitable thermoplastic resins for use in thepractice of the invention include polyamides, polyesters, celluloseesters, polyethylene, polypropylene, poly (vinyl chloride) or PVC, poly(vinylidene fluoride) or PVF₂, polyphenylsulfones and poly(tetrafluoroethylene) or PTFE. It is to be appreciated that as a resultof the typically lower processing temperatures to which resins aresubjected to in the practice of the invention, thermoplastic resins suchas polyphenylsulfones and PTFE are particularly suited for processing inaccordance with the invention.

Plasticization of PVC has been the conventional way to lower its meltviscosity so that it will flow sufficiently when heated to form acontinuous film. Nylon-11 and nylon-12 resins are representative of thepolyamides and cellulose acetate butyrate is an example of the celluloseesters contemplated for use in this invention. All of the suitablethermoplastic resins are available commercially from numerous sources.

The thermosettable resins which are suitable for this invention includeepoxy resins, polyurethanes, silicones, polyesters (includingunsaturated polyesters), acrylics, and hybrids such as epoxy-acrylic,polyester-acrylic, and epoxy-polyester. The glass transition temperature(T_(g)) of these resins must be high enough that the particles do notfuse together or sinter at temperatures likely to be encountered duringtransportation and storage. Preferably, the T_(g) is at least about 50°C.

The epoxy resins are those containing aliphatic or aromatic backboneswith oxirane functionality and are exemplified by the diglycidyl ethercondensation polymers resulting from the reaction of epichlorohydrinwith a bisphenol in the presence of an alkaline catalyst. Bisphenol A ismost commonly used but the bisphenols B, F, G and H are also suitable.By controlling the operating conditions and varying the ratio of thereactants, products of various equivalent weights can be made. For thepurposes of this invention, the epoxide equivalent weight (EEW) may befrom about 600 to about 2000 and the hydroxyl equivalent weight may befrom about 300 to about 400. These are available from a wide variety ofcommercial sources. The GT-series of bisphenol A epoxies fromCiba-Geigy, including 7004, 7013, 7014, 7074, and 7097 are examples ofuseful epoxy resins in this invention. Shell Chemical Co. also suppliessuitable epoxy resins under its EPON trademark.

Dicyandiamide, modified and substituted dicyandiamides, soliddicarboxylic acids and their anhydrides are examples of the many agentsthat may be used for the curing of epoxy resins. A curing agent in solidform is preferred for convenience in the formulation of epoxyresin-based powders as well as in the formulation of other resin-basedpowders in this invention.

Hydroxy functional polyesters are predominantly hydroxyl infunctionality; their acid number is preferably about 15 or less and,even more preferably, from about 1 to 2. The hydroxyl number, on theother hand, is preferably from about 25 to about 50, as conventionallyreported. The T_(g) is preferably higher than 50° C. because of itseffect on the blocking problem. They are the condensation products ofpolybasic carboxylic acids and polyhydric alcohols. Examples ofcarboxylic acids useful for the preparation of such polyester resins arephthalic acid, tetra- and hexahydrophthalic acids and their anhydrides,adipic acid, sebacic acid, terephthalic and isophthalic acids, 1,3- and1,4-cyclohexane-dicarboxylic acids, and trimellitic anhydride, esters ofsuch acids and mixtures of two or more. Ethylene-, diethylene-,propylene-, and trimethylene glycol exemplify the bifunctional alcohols,along with other dihydric alcohols such as hexanediol, 1,3-, 1,2-, and1,4-butanediols, neopentyl glycol, 2-butyl-2-ethyl-1,3-propanediol,2-methyl-1,3-propanediol, 1,4-cyclohexanediol, trimethylolpropane, andmixtures of two or more. Condensation of the acids and alcohols is awell-known reaction and various processes for carrying it out are alsowell known. The temperature is suitably from about 180° C. to about 300°C.; an azeotropic distillation with a solvent or a stream of an inertgas through a molten mixture of the reactants may be used to enhance theremoval of water formed by the condensation; and a catalyst such asp-toluenesulfonic acid or dibutyltin oxide may be used. An esterinterchange reaction catalyzed by a lead carboxylate or oxide, zincacetate, lithium hydroxide or carboxylate may be used at temperatures of200° C. to 300° C. Hydroxy functional polyesters are commerciallyavailable under the trademarks RUCOTE 107, CARGILL 3000, CARGILL 3016,and CRYLCOAT 3109.

The hydroxyl-functional polyesters are curable through the hydroxylgroups with aminoplasts and with aliphatic and aromatic isocyanates.Isocyanate curing forms resins which technically are polyurethanes butare often sold as polyesters. The aminoplasts are oligomers that are thereaction products of aldehydes, particularly formaldehyde, with amino-or amino-group-carrying substances exemplified by melamine, urea,dicyandiamide, and benzoguanamine. It is preferable in many instances toemploy precursors of aminoplasts such as hexamethylol melamine,dimethylol urea, and their etherified forms, i.e., modified withalkanols having from one to four carbon atoms. Hexamethoxymethylmelamine and tetramethoxy glycoluril exemplify said etherified forms.Thus, a wide variety of commercially available aminoplasts and theirprecursors can be used for combining with the linear polyesters of thisinvention. Particularly preferred are the amino cross-linking agentssold by American Cyanamid under the trademark CYMEL. In particular, theCYMEL 301, CYMEL 303, and CYMEL 385 alkylated melamine-formaldehyderesins are useful. Of course, it is possible to use mixtures of all ofthe above N-methylol products.

Aminoplast curing agents are generally provided in an amount sufficientto react with at least one-half the hydroxyl groups of the polyester,i.e., be present at least one-half the stoichiometric equivalent of thehydroxyl functionality. Preferably, the cross-linking agent issufficient to substantially completely react with all of the hydroxylfunctionality of the polyester, and cross-linking agents having nitrogencross-linking functionality are provided in amounts of from about 2 toabout 12 equivalents of nitrogen cross-linking functionality perequivalent of hydroxyl functionality of the polyester. This typicallytranslates to an aminoplast being provided at between about 10 and about70 phr.

The curing of hydroxyl-functional polyesters with an aminoplast takesplace in about 20-30 minutes at temperatures within the range of fromabout 120-200° C. (about 250-400° F.).

Acidic catalysts may be used to modify the curing of the polyester withan aminoplast resin by lowering the required temperature or raising thereaction rate or both. When it is desirable to lower the rate at ambientstorage temperatures, the acidic catalyst may be blocked with an amine.Volatile amines which may escape from the curing film when the catalystis unblocked by heat are suitable for this purpose. It is particularlydesirable for the purposes of this invention to delay full curing of thecomposition until the coated metal substrate has traveled aboutthree-fourths of the length of the curing oven. In a particularembodiment, the dwell time before full curing was about 33 seconds. Anamine-blocked dinonylnaphthalenesulfonic acid sold by King Industriesunder the trademark and number NACURE 1557 is an example of the blockedacid catalyst contemplated for use in the aminoplast curing of thepowder coating composition of this invention. The curing may also beretarded by the addition of free amines such as triethanolamine.

The diisocyanates cure the hydroxy-functional polyester resin by formingurethane linkages between the polymer chains at the hydroxyl groupsites. Aliphatic diisocyanates are exemplified by hexamethylenediisocyanate (HDI), diisocyanato dicyclohexylmethane (sold under thetrademark DESMODUR W by Miles Chemical), and isophorone diisocyanate(IPDI). Toluene diisocyanate (TDI) is an example of a suitable aromaticdiisocyanate. The low-temperature reactivity of free diisocyanates maybe lessened by adducting them with blocking agents selected from phenol,cresols, isononylphenol, amides such as ε-caprolactam, oximes such asmethyl-ethyl ketoxime and butanoneoxime, active methylenegroup-containing compounds such as diethylmalonate and isopropylidenemalonate and the acetoacetates, and sodium bisulfite. The adducts have aweak bond which breaks at an elevated temperature to regenerate theblocking agent and the free diisocyanate which can react with thepolyester in the desired manner. Examples of the blocked diisocyanatesinclude caprolactam blocked isophorone diisocyanate and caprolactamblocked hexamethylene diisocyanate. Examples of commercially availablecuring agents of this type are the 24-2400, 24-2430, and 24-2450products sold under the CARGILL trademark.

An excess of from about 10 to 20%, preferably 5 to 10%, by weight of thediisocyanate may be used beyond the stoichiometric amount. The reactionof the polyester with the diisocyanate is performed in the absence ofmoisture at a temperature of from about 80° C. to about 230° C. and,when a blocked diisocyanate is used, the temperature is preferably atleast about 120° C. and is more preferably about 200° C. or higher.Dibutyltin dilaurate and triethylene diamine are examples of thecatalysts that may be used to promote the diisocyanate cure. The use ofblocked isocyanates in the curing of coatings is described in a paperpresented by T. A. Potter, J. W. Rosthauser, and H. G. Schmelzer at theWater-Borne & Higher-Solids Coatings Symposium at New Orleans on Feb.5-7, 1986; the paper is incorporated herein by reference.

Carboxyl-functional polyesters are also suitable for the purposes ofthis invention. They may be made from the same polyfunctional acids andglycols as are the hydroxyl-functional polyesters but with an excess ofthe acids. The acid number is from about 18 to about 55. They areexemplified by products sold under the trademarks CRYLCOAT 430, CRYLCOAT3010, URALAC 3400, URALAC 3900, and GRILESTA V7372, which has a T_(g) of60° C. and an acid number of 32-35, and which is sold by Ems-Chemin AG.Fast cures are achieved with polyepoxide curing agents such astriglycidyl isocyanurate (TGIC).

Unsaturated polyesters suitable for use in the practice of the inventioninclude ethylenically unsaturated reaction products of an organic di orpolyfunctional acid and a di or polyfunctional alcohol. Typically theacid is unsaturated. Such polyester resins typically work best incombination with a copolymerizable second resin such as diallylphthalate. Initiators may also need to be incorporated.

A hybrid resin system is typically considered a mixture of acarboxyl-functional polyester and an epoxy resin. The acidic polyestersuitably has an equivalent weight of 550-1100 and the epoxy resin has anequivalent weight of 600-1000. Zinc oxide is effective as a catalyst at1-5 parts per hundred parts by weight of the resins to improve the curerate and physical properties of the product. Other hybrid resin systemssuch as the epoxy-acrylic and polyester-acrylic mixtures mentioned aboveare also suitable for this invention.

The preferred acrylic resins for coating powders are copolymers of alkylacrylates and/or methacrylates with glycidyl-methacrylates and/oracrylates and olefinic monomers such as styrene. Glycidyl-functionalacrylic resins are sold by Mitsui Toatsu Chemicals, Inc. under thetrademark ALMATEX (e.g., PD-7610, PD-7690, PD-6100). The ALMATEX PD-7610resin, for example, has an epoxy equivalent of 510-560 and a melt indexof 50-58. Solid dicarboxylic acids having, for example, 10 or 12 carbonatoms are used to cure the glycidyl-functional acrylic resins. Acarboxy-terminated polymer may also be used as a cross-linking agent forsuch acrylic resins. Hydroxyalkyl acrylate and methacrylate copolymersare also suitable for this invention.

Suitable silicone resins for use in this invention should be solid atroom temperature and preferably have a T_(g) of at least about 45° C.The organic moieties of the silicone resins are aryl, particularlyphenyl, or short chain (C₁ -C₄) alkyl. For good heat resistance, methyland phenyl groups are the organic moieties of choice. Generally, themore phenyl groups, the higher heat-resistance provided. Examples ofsuch silicone resins are phenylsilicone SY-430, sold by Wacker Silicone,Conshohocken, Pa., having an average molecular weight of about 1700,methylsilicone MK also sold by Wacker and methylphenylsilicone 6-2230sold by Dow Corning.

For high temperature stability, silicone resins useful in the inventionhave a degree of substitution as described in Silicones in ProtectiveCoatings, supra of about 1.5 or less, typically between about 1 andabout 1.5. Specifically, degree of substitution is defined as theaverage number of substituent groups per silicon atom and is thesummation of the mole per cent multiplied by the number of substituentsfor each ingredient. Silicon resins are used which self-condense at highend-use temperatures, e.g., that of a barbecue grill or an automobileexhaust part. This requires siloxane functionality (Si--O--H), andsilicone resins used herein have an --OH content of between about 2.5and about 7.5 wt % of the silicone resin. Suitable silicone resins foruse in the invention are discussed in "Silicones in Protective Coatings"by Lawrence H. Brown in Treatise on Coatings Vol. 1, Part III"Film-Forming Compositions," pp. 513-563, R. R. Meyers and J. S. Long,eds., Marcel Dekker, Inc. New York, 1972, the teachings of which areincorporated herein by reference. Suitable silicone resins are alsodescribed in U.S. Pat. Nos. 3,170,890 and 4,879,344 3,585,065 and4,107,148, the teachings of which are incorporated herein by reference.

Further, crystalline resins, such as the crystalline polyester PIONEERPIOESTER 4350-55, can also be used.

Additives suitable for inclusion in the coating powder compositionsinclude antioxidants, light stabilizers, pigments and dyes, processingaids, antiblocking agents, and the anti-cratering agents.

Examples of antioxidants include, but are not limited to: hinderedphenols, phosphites, and propionates. Examples of hindered phenols are1,3,5-trimethyl-2,4,6-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)benzene;octadecyl-3-(3,5-ditert-butyl-4-hydroxyphenyl)propionate; tetrakis[methylene-3(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate]methane);4,4'-butylidene-bis(5-methyl-2-t-butyl)phenyl; and2,2'-ethylidene-bis-(4,6-di-tert-butylphenol). Examples of phosphiteantioxidants are tris(2,4-di-tert-butyl-phenyl)phosphite;bis(2,4-di-t-butyl-phenyl) pentaerythritol diphosphite; and2,2'-ethylidene-bis(4,6-di-t-butylphenyl)fluorophosphite. Examples ofpropionate antioxidants are dilauryl thiodipropionate and distearylthiodipropionate. IRGANOX 1010 hundred phenol and IRGAFOS 168 phosphiteare commercially available antioxidants. Antioxidants may be used inamounts ranging from about 0.01 to about 2.0 percent by weight of thepowder.

Light stabilizers and UV absorbers are exemplified by benzophenonestabilizers, such as those sold under the trademarks CYASORB-UV 2018(American Cyanamid), hindered amine compounds, including those marketedby Ciba-Geigy under the trademarks TINUVIN 144, TINUVIN 292, TINUVIN944, TINUVIN 622LD, and TINUVIN 770(N,N-diphenyl-N,N-di-2-naphthyl-p-phenylene-diamine), and BASF's UVINULM40 and UVINUL 490, particularly those containing tetraalkyl-piperidinyl functionality, and UV absorbers marketed by Ciba-Geigyunder the trademark TINUVIN 900 and by American Cyanamid under CYANOX3346.

Examples of antiblocking agents (dry flow agents) are fumed silica,clay, talc, fumed alumina, and precipitated silica. Commercial examplesof antiblocking agents are sold under the trademarks AEROSIL andCABOSIL. Flow levelling (anti-cratering) agents are sold under thetrademarks TROY EX-486 and RESIFLOW P-67 (a low molecular weight acrylicresin). Other additives often used to de-gas the films are sold underthe trademarks URAFLOW B (benzoin), OXYMELT A-1 and OXYMELT A-2.

The deleterious effects of surfactants and other additives such as chainstoppers in the final coating film can be avoided by not including suchadditives in the powder coating compositions processed in accordancewith the invention. It is to be understood, however, that at least incertain specific embodiments, it may be desirable to use a surfactantthat enhances the solubility of the selected resin in the selectedprocess fluid media. For example, fluorocarbons, fluoroethers, andsiloxanes can serve as useful surfactants in combination with a processmedia fluid of carbon dioxide.

It is to be appreciated that the systems of the invention, such as theabove-described continuous processing systems, can be relatively easilypurged to clean between processing runs for or with different productcompositions. Thus, such systems can have more desirable commercialutility and application.

It is also to be appreciated that an advantage of to the continuousprocessing of powder coating materials in accordance with the inventionis the facilitation of the attainment of steady state processingconditions such as temperature, pressure and time at conditions.

A wide variety of powder coating materials can be prepared in accordancewith the invention, including:

a.) the powder coating materials identified and described in thepreviously referred to commonly assigned patent applications Ser. No.08/662,104, filed Jun. 14, 1996 and Ser. No. 08/354,308, filed Dec. 12,1994, the disclosures of which are fully incorporated herein byreference, including the cellular, generally spherical coating powderparticles and such as produced by the dissolution of the ingredients ofa coating powder in a supercritical fluid with a co-solvent, asdisclosed therein. Such powder coating materials are disclosed thereinas having an extremely narrow particle size distribution. Specifically,with the exception of a minor amount of fines having particle diameterof less than 2, the particle sizes of such powder materials are allwithin the range of from about 2 to about 40 microns, with about 96% ofthe volume of the powder has a particle size of 20 microns or less andabout 75% of its volume has a particle size between 2 and 20 microns.Thus, such powder coating particles range in size from less than 2 toabout 40 microns with a mean particle size of about 4.4 microns and amedian size of from about 6 to about 7 microns, wherein particle sizemeasurement are made with a COULTER LS Particle Size Analyzer wherein aFraunhofer optical model (PIDS included) and an LS 130 fluid module isused;

b.) the powder coating materials identified and described in U.S. Pat.No. 5,399,597, issued Mar. 21, 1995, the disclosure of which is fullyincorporated herein by reference, including the flake-type and roundedparticles disclosed therein such as produced by a method of:

providing a first vessel connected by piping to a second vessel;

charging such first vessel with starting materials;

supplying CO₂ to such first vessel and holding such CO₂ in such firstvessel at such a temperature and pressure that such CO₂ comprises asupercritical fluid;

agitating such starting materials and such supercritical fluid;

transferring such CO₂ and such starting materials through a spray nozzlehaving an orifice diameter of from about 0.001" to about 1"; and then

discharging such CO₂ and such starting materials into a second vesselbeing maintained at a lower pressure than the first vessel;

c.) the powder coating materials identified and described in U.S. Pat.No. 4,582,731, issued Apr. 15, 1986 and U.S. Pat. No. 4,734,227, issuedMar. 29, 1988, the disclosures of which are fully incorporated herein byreference, including particles in a narrow size distribution, havingaverage sizes ranging from 0.3 micron to about 3 microns;

d.) powder coating materials which include crystalline resins such asPIONEER PIOESTER 4350-55 and a curing agent; and

e.) highly reactive powder coating system, such as primary amine curedepoxies, suitable for application on temperature sensitive substratessuch as plastics, wood, and pre-assembled articles that contain orinclude heat sensitive components.

In general, the powder coatings prepared in accordance with theinvention are suitable for application to a wide variety of substratematerials including metallic and non-metallic substrates. For example,such powder coatings can be applied to various metallic substrates whichare inert to the coating material. Such metallic substrates cantypically include various structural metals such as iron, steel andaluminum, for example. Suitable non-metallic substrates can include woodand paper-based substrates including particle board and cardboard,glass, ceramics, plastics and rubber, for example.

The present invention is described in further detail in connection withthe following examples which illustrate/simulate various aspectsinvolved in the practice of the invention. It is to be understood thatall changes that come within the spirit of the invention are desired tobe protected and thus the invention is not to be construed as limited bythese examples.

EXAMPLES Comparative Example 1 (CE 1) and Example 1 (Ex. 1)

Ethylene acrylic acid resin was processed in a 27 mm co-rotatingextruder having a two-strand die under the conditions identified inTable 2 below, with the processing of Ex. 1 done in accordance with theinvention with the addition of CO₂ to the processing extruder while inCE 1 the resin was processed through the extruder without the additionof such process media fluid.

                  TABLE 2                                                         ______________________________________                                                          CE 1     Ex. 1                                              ______________________________________                                        Melt Temperature (° F.)                                                                  325-400  324                                                  Melt Pressure (psi) 400-600 210                                               CO.sub.2 Injection Pressure (psi) -- 800-1000                                 CO.sub.2 Feed Rate (lbs/hr) -- 3                                              Output Rate (lbs/hr) 30 30                                                  ______________________________________                                    

The so formed powder coating compositions were then pelletized. Therespective powder coating composition pellets were then cryogenicallyground in a Retsch Ultra Centrifugal Mill. More specifically, therespective pellets were immersed in liquid nitrogen and fed to the millwith the entire product being ground. The mill included a 12 pin rotorand a screen with 1.0 mm openings. The product yield was the percentageof the amount of the originally provided resin which formed product thanwas ground to less than 250 microns (60 mesh), see TABLE 3, below.

                  TABLE 3                                                         ______________________________________                                                           CE 1   Ex. 1                                               ______________________________________                                        Product Yield (%)  12     31                                                  ______________________________________                                    

Discussion of Results

As shown in TABLE 3, a significantly higher yield was obtained when thethermoplastic-based powder coating was processed in accordance with theinvention.

In addition, the grinding of pellets of conventionally preparedthermoplastic composition (CE 1) tended to result in extensiveelongation rather than fracturing, with the particles being formedhaving "tails." It will be appreciated that the occurrence or presenceof such tails can result in poor fluidization and handling duringapplication of such a powder coating composition.

In contrast, the thermoplastic composition prepared in accordance withthe invention (Ex. 1) appeared to have a higher tendency to fracturerather than tear and form a tail. As a result, such a preparedcomposition can facilitate handling and application.

Comparative Example 2 (CE 2) and Example 2 (Ex. 2)

In this comparative example and example, a hybrid powder coatingformulation (i.e., a formulation containing both epoxy resin andcarboxyl-functional polyester resin and specifically identified in TABLE4, below) was extruded through a sheet die and then ground in the mannerdescribed below. More specifically, the hybrid formulations wereprocessed in a 27 mm co-rotating extruder and discharged through a sheetdie under the conditions identified in Table 5 below, with theprocessing of Ex. 2 done in accordance with the invention with theaddition of CO₂ to the processing extruder.

                  TABLE 4                                                         ______________________________________                                                                          WT.                                           INGREDIENT IDENTIFICATION SUPPLIER PERCENT                                  ______________________________________                                        Epon 2002                                                                              Bis A Epoxy  Shell Chemical                                                                            24                                            Rucote 551 Carboxyl Polyester Ruco Chemical 24                                P-67 Polyacrylate flow Estron Chemical 0.7                                     modifier                                                                     Benzoin Flow modifier DSM 0.4                                                 DT-3329 Matting agent Ciba-Geigy 1.6                                          R-902 Titanium oxide DuPont 39                                                Minex Silicate filler Indusmin 10                                             Toner Pigments  0.3                                                         ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                                             CE 2   Ex. 2                                             ______________________________________                                        Melt Temperature (° F.)                                                                     120    111                                                 Melt Pressure (psi) 2480 1320                                                 Extrudate Temperature (° F.) 255 222                                   CO.sub.2 Injection Pressure (psi) -- 1300                                     CO.sub.2 Feed Rate (lbs/hr) -- 2.4                                            Output Rate (lbs/hr) 50 50                                                  ______________________________________                                    

The so formed powder coating composition sheets were then flaked. Therespective powder coating composition flakes were then fed to the millwith the entire product being ground. The product yield was thepercentage of the amount of the originally provided resin which formedproduct which passed through a 140 mesh screen.

The extrudate temperature was measured using an optical pyrometer.

The flowability of the powder coating material resulting from Ex. 2 wasalso compared to that of the powder coating material resulting from CE 2in the following manner:

1. A cylindrical pellet, 1/2" (12.7 mm) in diameter by 6 mm in length,was pressed from the material being tested.

2. The pellet was then pressed, using as little pressure as possible andreleasing immediately, onto a hot (375° F.) electric cure plate set at a35° angle.

3. The pellet was then allowed to melt. At a time interval of fiveminutes after the pellet first contacted the plate, the length of themelt flow was measured using a steel rule. The length of the respectivemelt flows for CE 2 and Ex. 2 are provided in TABLE 6 below.

                  TABLE 6                                                         ______________________________________                                                           CE 2   Ex.2                                                ______________________________________                                        Product Yield (%)  50     74                                                    Melt Flow (mm) 25 35                                                        ______________________________________                                    

Discussion of Results

It is evident from the large pressure drop in melt pressure that theaddition of the CO₂ in Ex. 2 is reducing the viscosity of the resinbeing processed which in turn reduces the pressure at the die as thelower viscosity material can more easily exit through the sheet die. Inaddition, the higher yield obtained in Ex. 2, compared to that obtainedin CE 2, demonstrates that the material processed in accordance with theinvention was more easily processed.

The flow data confirms that the reduction in temperature associated withthe practice of the processing of the invention (e.g., addition of CO₂as a process media during extrusion) will reduce the extent of curereaction or B staging in the extruder and result in a product havingbetter flow characteristics, which in turn generally translates into asmoother coating.

Example 3 (Ex. 3)--Processing Crystalline Materials

In accordance with the invention, a crystalline thermosetpolyester-containing formulation (identified in TABLE 7, below) wasprocessed through an extruder under the conditions defined in TABLE 8below.

                  TABLE 7                                                         ______________________________________                                                 IDENTI-                 WT.                                            INGREDIENT FICATION SUPPLIER PERCENT                                        ______________________________________                                        Pioester Unsat. crystalline                                                                        Pioneer     77                                              polyester                                                                    P-67 Polyacrylate flow Estron Chemical 1.0                                     modifier                                                                     Luperco Peroxy ketal Pennwalt 1.5                                             231-XL                                                                        Irgacure 184 Acetophenone Ciba-Geigy 0.8                                      R-902 Titanium dioxide DuPont 19.7                                          ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                                           Ex. 3                                                      ______________________________________                                        Melt Temperature (° F.)                                                                   94                                                           Melt Pressure (psi) 950                                                       CO.sub.2 Injection Pressure (psi) 800-1000                                    CO.sub.2 Feed Rate (lbs/hr) 1                                                 Output Rate (lbs/hr) 30                                                     ______________________________________                                    

Discussion of Results

This example demonstrates that the invention permits and facilitates theextrusion processing of crystalline materials.

It is to be appreciated that conventional extrusion processing typicallyis not useful in connection with crystalline materials. In conventionalextrusion processing, it is believed that as the crystalline materialmelts, the viscosity of the material rapidly decreases rendering thematerial difficult or incapable of being so handled or processed. Incontrast and in accordance with the invention, it is theorized that theaddition of the process media fluid reduces the viscosity of the processstream and thus permits the material to be processed through theextruder.

Thus, the continuous processing of powder coating compositions inaccordance with the invention can afford a number of advantagesincluding, for example:

a.) providing improved product consistency;

b.) providing improved processing of fast curing compositions;

c.) simplify, reduce or eliminate grinding processing required to formthe final powder product form;

d.) facilitate formation and maintenance of seals required in and forsuch processing apparatus, thus avoiding the problems associated withprior art high pressure operation, such as identified above;

e.) providing desired processing flexibility where, for example, theprocess media can be added at one or more locations, as desired;

f.) providing simple and effective removal of the process media fluidfrom the powder coating precursor;

g.) permitting processing, such as occurs within an extruder, atrelative low processing temperatures to permit and facilitate the use oftemperature sensitive raw materials;

h.) preventing undesired curing of thermosettable resins in areas suchas the pump seal area;

i.) allowing utilization of process media without any significantsolubilization of the resin or other composition component;

j.) allowing utilization of process media having associated therewithsignificant solubilization of the resin or other composition componentsuch as to form low viscosity solutions which, for example, can besprayable such as to form regular spheres;

k.) allowing processing with co-solvents; and

l.) providing a more easily friable foamed extrudate, capable of beingreduced more controllably in particle size than typical flake extruderproduct.

The invention illustratively disclosed herein suitably may be practicedin the absence of any element, part, step, component, or ingredientwhich is not specifically disclosed herein.

The foregoing detailed description is given for clearness ofunderstanding only, and no unnecessary limitations are to be understoodtherefrom, as modifications within the scope of the invention will beobvious to those skilled in the art.

What is claimed is:
 1. In a method for producing a powder coatingmaterial, the step of: feeding a powder coating precursor streamcomprising powder coating ingredients including at least one resin andat least one additional powder coating ingredient into a continuousextruder and contacting said powder coating precursor stream with aprocess media fluid effective to reduce the viscosity of the powdercoating precursor stream to allow processing of the powder coatingprecursor stream at a temperature below the softening temperature of theresin ingredient of the powder coating composition under preparation toform an extrudate stream, said process media fluid comprising a processmedia material in the form of a fluid selected from the group consistingof supercritical fluids and liquified gases wherein, subsequent to saidcontacting step, said method additionally comprises the step of heatingsaid extrudate stream contacted with the process media fluid.
 2. Themethod of claim 1 wherein said heating step comprises passing the powdercoating precursor stream contacted with the process media fluid througha heated nozzle.
 3. The method of claim 1 wherein said resin iscrystalline.
 4. In a method for producing a powder coating whereinpowder coating raw materials including at least one resin and at leastone additional powder coating ingredient are fed to and processed in acontinuous extruder to disperse the at least one additional ingredientwith the at least one resin to form an extrudate product, the stepsof:adding a process media fluid comprising a process media material inthe form of a fluid selected from the group consisting of supercriticalfluids and liquified gases to a process stream of at least one of thefollowing:a). the raw materials fed to the continuous extruder; b). theraw materials processed in the continuous extruder; and c). theextrudate product of the continuous extruder, said addition beingeffective to reduce the viscosity of the selected process stream toallow processing of the process stream at a temperature below thesoftening temperature of said resin ingredient of the powder coatingunder preparation; and heating said process stream in contact with saidprocess media fluid.
 5. The method of claim 4 wherein said powdercoating precursor stream is processed in the continuous extruder at atemperature no greater than the softening temperature of the resin. 6.The method of claim 5 wherein said powder coating precursor stream isprocessed in the continuous extruder at a temperature of at least about10-20° F. below the softening temperature of the resin.
 7. The method ofclaim 5 wherein said powder coating precursor stream is processed in thecontinuous extruder at a temperature of at least about 20-40° F. belowthe softening temperature of the resin.
 8. The method of claim 4additionally comprising the step of adding a co-solvent to a processstream comprising at least one of the following:a.) the raw materialsfed to the continuous extruder; b.) the raw materials processed in thecontinuous extruder; and c.) the extrudate product of the continuousextruder, and said co-solvent is subsequently removed along withvolatiles including said fluid process media.
 9. The method of claim 4wherein said process media fluid comprises carbon dioxide.
 10. A methodfor producing a powder coating, said method comprising the stepsof;extruding a premixed blend of powder coating raw materials includingat least one thermosettable resin and at least one curing agent for theat least one thermosettable resin to form an extrudate product; feedinga stream of the extrudate product through a melt pump to form a streamof extrudate product at increased pressure; and spray drying the streamof extrudate product at increased pressure to form the powder coating,wherein at least one of the blend of powder coating raw materialsundergoing extrusion and the stream of extrudate product at increasedpressure is contacted with a process media fluid selected from the groupconsisting of supercritical fluids and liquified gases, the processmedia being effective to reduce the viscosity of the materials of theselected process stream to allow processing at a temperature below thesoftening temperature of said thermosettable resin ingredient of thepowder coating under preparation; and heating said process stream incontact with said process media fluid.
 11. The method of claim 10wherein said powder coating precursor stream is processed in acontinuous extruder at a temperature no greater than the softeningtemperature of the resin.
 12. The method of claim 10 wherein said powdercoating precursor stream is processed in a continuous extruder at atemperature of at least about 10-20° F. below the softening temperatureof the resin.
 13. The method of claim 10 wherein said powder coatingprecursor stream is processed in a continuous extruder at a temperatureof at least about 20-40° F. below the softening temperature of theresin.
 14. The method of claim 10 wherein said process media fluidcomprises carbon dioxide.
 15. The method of claim 10 additionallycomprising the step of applying an additional quantity of the processmedia fluid to seals in said melt pump to keep such seals free of thepowder coating materials being processed.
 16. The method of claim 4additionally comprising the step of statically mixing the extrudateproduct of the continuous extruder.
 17. The method of claim 10additionally comprising the step of statically mixing the process mediafluid with the stream of extruded product at increased pressure.